Electron beam lithography systems have been built in which an electron beam is projected vertically downwards onto a substrate which is mounted on a stage that is equipped to move vertically, and in both directions in a horizontal plane. Electric and magnetic fields are produced in electron optics in order to focus the electron beam vertically downwards on to the substrate. The electron optics also controls the deflection of the beam in order to allow for small deflections of the electron beam in the horizontal plane. One problem encountered with such electron beam devices is that the vertical distance between the electron optics and the substrate must be maintained substantially constant at a predetermined distance in order to maintain the focus of the electron beam on the substrate. Such a vertical distance, or height of the substrate, is also critical in order to insure that calibration and precision are preserved for the electron beam deflection produced in a horizontal plane by the electron optics. The problem of substrate height control is accentuated by distortions or flatness imperfections in the substrate. Such distortions may be caused by warping of the substrate during the chemical or thermal processing steps used in semiconductor device fabrication. The distortions may cause different areas of the substrate to be at different vertical heights when mounted on the electron beam lithography stage.
A variety of sensor devices have been used in order to detect the substrate height; that is, the vertical distance between the electron optics and the substrate. Such sensors have included light (photon) optics, capacitive sensors, and inductive sensors. A drawback with such prior sensors is that they are generally so large in size as to be cumbersome, and are expensive.
When an electron beam bombards a substrate, it produces high energy backscattered electrons and low energy secondary electrons which travel outwards from the impact region on the surface of the substrate. The backscattered electrons have approximately the same energy as bombardment electrons, and their energy is generally greater than one thousand volts.
The secondary electrons generally have an energy in the range of five to twenty volts.
The backscattered electrons travel in substantially straight lines. The flux of backscattered electrons is defined by a backscatter coefficient which varies with the composition and texture of the substrate upper surface. The backscattered electron flux also varies with the bombardment beam current. The intensity of the backscattered electron flux also varies with substrate irregularities such that localized sloping of the substrate upper surface causes backscattered electron flux to be directed toward or away from the direction of electron bombardment.
It is important not only that the substrate height be detected, but also that a method for adjusting the stage vertical position also be provided in order to move the substrate to a height satisfying the critical requirements of the electron beam system.